This is an article I have put together from many sources.

I hope it answers all of your Duraspark Timing and Tuning questions.

Lets start at the beginning......

Maybe you've just built up a brand new engine, or upgraded to new heads and a cam, perhaps you're simply trying to dial-in an existing combination. In either scenario, one area of tuning that is highly overlooked and greatly misunderstood is timing. All too often we see people dropping in their distributor, making a quick adjustment with their timing light, and setting off to make another pass.

Timing is everything, and without a proper timing curve, every thing else goes out the window. Jetting changes, fuel pressure adjustments, are all useless if first the timing is not set correctly.


So what is timing? In a nutshell, timing or 'ignition timing' relates to when the sparkplug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to melted pistons.

Why an engine needs more advance as its speed increases.
When the compressed mixture inside a cylinder is ignited it takes time for the flame front to reach the piston and for the expanding gases to start pushing it down. The time that this takes changes according to a number of variables such as mixture strength, how well the cylinder has filled (dependent on volumetric efficiency and throttle opening), compression ratio and combustion chamber shape. Given the same circumstances of mixture strength, cylinder filling and CR, the time taken for the mixture to fully ignite and burn is the same regardless of engine speed. At increasingly higher RPM however, the time available for this burn to take place is correspondingly less, so it follows that you have to start burning the mixture earlier in order for it to push on the piston at the right time. This is the basis for increasing ignition advance.
Too much of this and the burning mixture hits the piston as it rises (pinking or pinging), too little and the flame front reaches the piston far too late and does not do a good job of pushing the piston down and the engine behaves like a herd of turtles. One of the reasons a diesel engine does not perform at higher RPM is that it has compression only ignition, so there is no way to increase the effective ignition advance.
How this is achieved
The distributor as fitted to conventional ignition systems does not just distribute the spark amongst the cylinders and switch the coil; it also contains a centrifugal mechanism that advances the ignition timing automatically as engine RPM rises. Normally there are a pair of weights within the distributor which under the affects of centrifugal force tend to be thrown outwards, this tendency is greater as RPM increases. The weights are shackled by two small springs that restrain them progressively. As the weights move outwards they exert a turning force on the top of the distributor shaft relative to the driven part of the shaft, this moves in the same direction as the distributors rotation thereby causing the points/electronic trigger to actuate earlier and advancing the ignition timing. As engine speed increases the weights overcome more of the spring's tension and advance the timing still more. There is normally a stop of some kind that limits the amount of advance that the distributor can supply. This centrifugal mechanism is usually hidden away underneath the baseplate of the distributor.


In this article we're going to focus primarily on carburated, non-computer controlled, engines which have fully adjustable distributors. The EEC-IV computer controlled Fords allow for setting initial timing, but the rest is adjusted by the computer. The newer modular engine Fords have distributor-less ignitions which offer no adjustability from the factory, although companies like Steeda have recently developed timing adjusters for these engines. Some Fords, particularly in the 70's and early 80's, had distributors where timing was fixed due to emissions reasons.

When it comes to timing the most common myth is that adjusting the timing simply means moving the distributor clockwise or counterclockwise. While this does affect the timing, it is not the correct way to adjust the timing curve. To explain why, we first we need to define some terms.

Advancing and retarding timing refers to increasing or decreasing the 'time' at which spark is delivered to the cylinders. This 'time' is measured in crankshaft degrees, signified by marks on the harmonic balancer, and a reference pointer on the block or timing chain cover. When the piston is at Top Dead Center (TDC), this is synonymous with zero degrees on the balancer. Ten degrees before that point would mean the piston is ten degrees of rotation from being at TDC.

So how does the crank position relate to the distributor?
The distributor shaft on Ford engines is driven by the camshaft gear, which is turned at half-crank speed by the timing chain connected to the crankshaft. Thus there is a direct correlation between the position of the crank and the position of the distributor. Remember, the distributor is a switch. Regardless of the type of distributor you have, there is a fundamental design common to all of them; the shaft is in a fixed position, spinning in direct relation to the crankshaft. On the shaft sits the trigger which activates the switch. On electronic distributors the trigger may be a magnetic sleeve with eight openings, or in the case of points, its simply an arm that open and closes the points. The distributor housing does not spin and it contains the actual switch, such as the Pertronix box, which is mounted on a breaker plate. By rotating the housing you in effect move the position of the switch, changing when it triggers a spark. When you rotate the distributor to "adjust the timing" you are moving the switch on the housing side in relation to the trigger on the shaft.

Rotating the distributor housing clockwise on a Ford advances the timing (i.e. spark is being fired a greater number of degrees before the piston reaches TDC), and counterclockwise decreases the timing.

When referring to timing, there are really four terms that must be considered; initial timing, mechanical (or centrifugal) timing, total timing, and vacuum advance. There is also cam timing which is more appropriately termed valve timing, since it deals with when the valves open and close in relation to crank position. We won't talk about this since it has no dynamic bearing on ignition timing.

Initial: This is the most common adjustment that people associate with timing. At idle, with the vacuum advance hose disconnected and plugged, this is the timing that you would see if you flashed timing light on the timing marks. On typical stock engines you'd see as low as 0 to as high as 15 degrees. Most Ford shop manuals specify around 6-8 degrees initial timing advance for the 289-351 motors.

Mechanical/Centrifugal: Most V8 distributors contain an internal advance mechanism consisting of two each of weights, springs, and slotted 'reluctor' arms. There is also a stop tab for the arms. On Fords this assembly can only be seen by removing the cap, rotor, and breaker plate; we'll get to removal a bit later. As the distributor shaft spins with increasing rpms, the centrifugal force acts on the weights, which begin to force outwards against the springs. This movement rotates the shaft and thus advances the timing. The slotted arm controls how much the weights can move the assembly, and the springs control how fast the assembly reaches that limit. The reluctor arm on a Ford has two slotted sides, only one side contributes to the timing, the arm can be flipped around if more advance is needed (see pictures.) On Fords each side is stamped with a number, usually 10L and 13L; or some have 15L and 18L. These numbers refer to 1/2 of the total degrees of timing that will be obtained when using that arm. So for example a 15L arm would contribute 15 x 2= 30 degrees of timing when full against the stop.

Total Advance: So far we have looked at initial advance and mechanical advance. Both of these combined gives total advance. Say for example initial was found to be 6 degrees, and we visually verified that the reluctor arm was on the 15L side. Total timing, theoretically, is then the initial + mechanical. In this case 6 + (15 x 2) = 36 degrees. If we shined a timing light on the marks (with vacuum hose disconnected and plugged), at idle we'd see 6 degrees, then as we increased the engine speed, we'd see more and more advance, until at some point the total centrifugal advance would be reached, and we would see 36 degrees. When exactly the total advance occurs is of great importance when it comes to performance, and we discuss this in the section below on "curving."

Vacuum Advance: Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpms, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

In short, vacuum advance was developed to optimize fuel economy and reduce emissions. It is not a bad thing to have on a car which sees a lot of street driving, and in such conditions the engine will perform better with it properly adjusted. However many factory and aftermarket performance distributors do not even come with a vacuum advance. The reason is simply because race cars do not spend much time at part throttle.

Curving for Performance
A timing curve is simply a plot of how much ignition advance takes place over the rpm range. In other words, when the timing advances is just as critical as how much it advances.

When it comes to performance there are many different engine combinations, buildups, components, and uses….Each requiring slightly different timing curves. On the other hand if you have a stock motor, and do not care for every extra horsepower, you really do not need to do more than follow the shop manual procedures. However even a stock or mild daily driver motor can be made to accelerate faster with a five minute timing curve adjustment.

The rule of thumb is that the higher the compression ratio, the less total timing it can handle before detonation, and also the higher octane rating it needs to control detonation. Low octane fuels ignite faster, thus require less timing advance. Conversely high octane fuel can handle slightly more advance. Dyno testing has shown that most small block Fords with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance. Engines with 9.5:1 - 10.5:1 run best with 35-38 degrees total, and above 11:1, should not go higher than 35 deg. total. When using power adders such as nitrous, super or turbo chargers, the timing should be advanced accordingly.


The first step in curving a distributor is to set your initial and total advance. As detailed above and in the picture captions, the total is determined by the reluctor arm setting plus the initial advance. Ideally you should keep the initial between 10 and 20 degrees, and the total in the ranges listed above for your compression ratio. For example, if you are shooting for 40 degrees total, and your reluctor arm is on the 15L slot, you would have 30 degrees mechanical advance, requiring the initial to be set at 10 degrees.

The second step is to dial-in how fast the distributor achieves the total advance. This is controlled by the springs which hold back the weights, under the breaker plate. A stock distributor usually has one light and one heavy spring, and brings the timing in really slow, such that the distributor may only reach the total timing at very high engine speeds, 4500+ for example. For performance driving, the best acceleration comes when total advance is achieved within the range of 2000rpm to 3000 rpm. To adjust this rate, you can replace the stock springs with lighter tension springs. You can also bend the tabs on which the springs connect to change their tension.


Once you've set the initial and mechanical timing, and adjusted the curve, you should be very very close, if not right at, the optimum timing curve for wide-open throttle performance. You should use timing light at this point to confirm that the initial timing is where you set it, and steady, and then check the timing from idle to 3500 in 500rpm increments. The curve should increase a few degrees at every checkpoint until 2500-3000rpm, where it hits the maximum. After 3000 it should not go beyond the total advance.


So How The Hell Do I Know How Much Advance I Need???

Read On….

Establishing Maximum Advance Requirement
Notwithstanding the compression ratio and other factors, the characteristic that determines the maximum advance setting is the shape of the combustion chamber and the position of the spark plug.

Combustion Chamber Shape and Spark Plug Location

Combustion chambers and spark plug location and the number of plugs will have a marked effect on the time required to complete the combustion process. A large open chamber like a hemi which has a high surface to volume ratio, will combust more slowly than a wedge or modern pentroof chamber simply because it has more cold, metal molecules in contact with the combustion gasses which tends to slow reaction rates. For this reason, these chambers will require that the spark be initiated sooner to achieve PCP at the correct time.

The slowest combusting chamber would be an open chamber with a large bore size and the spark plug at one edge of the chamber. The flame front has a long distance to cover to complete combustion. By placing the plug in the center of the chamber, you halve the distance that the front needs to travel and will be able to reduce the spark advance needed to achieve maximum power. Another solution would be to add another spark plug to create two flame fronts which would also require much less time to combust. This is the solution in most aircraft engines where big bores and poor fuel distribution and homogeny require solutions to increase ignition probability.

Modern 4 valve engines with shallow pentroof chambers and a central plug location are fast, efficient combustors, requiring minimal advance for maximum power.


Most small block fords use a heart shaped combustion chamber. These require 36-38 degrees. Factor in your compression ration from above and come up with a number in the middle. These few degrees difference can be made up for with a small adjustment to the initial timing to save from modifying the reluctor arms again.


Establishing static advance requirement
The static advance requirement for a modified engine is very much dependent on the duration of the cam fitted. Below is a table of advance requirements and expected idle speeds for a range of cam specifications. ON NO ACCOUNT use these settings before the maximum advance on the distributor has been correctly limited.

Cam duration-----Idle speed expected------------------Advance
----270------------------600-800----------------------------10-12
----280------------------900-1000---------------------------12-14
----290------------------1000-1100-------------------------14-16
----300------------------1100-1200-------------------------16-18
----310+----------------1100-1400-------------------------18-20

When establishing static advance the golden rule is never use less than 10; never use more than 20 degrees. The engine may well tolerate more than 20 degrees at idle, but the moment the throttle is opened and cylinder filling is improved it will ping heavily. One problem often encountered when using more static advance than standard is that the engine may 'kick-back' when starting causing the starter to slow dramatically, this can be confused with a flattened battery or worn starter motor. You may need to compromise by the odd degree or two if your engine will not tolerate the required degrees of advance at start-up.
Static advance implies a measurement taken when the engine is stationery, however it is usually set at idle in order that any latency in the distributor drive gear is taken up. A rough setting can be made when the engine is still, but it MUST be set at 1300RPM or lower with the vacuum advance disconnected so that any latency is taken up and the centrifugal advance has not yet started its operation.

Establishing mechanical advance requirement
We have our desired static and maximum advance figures already calculated, so now we can use the same simple formula to establish how much centrifugal advance we need from the distributor.
Example:
Maximum advance 38 degrees, required static advance 18 degrees (38-18) = 20 degrees required.
In our example the standard distributor is designed to give maximum advance from a starting point of say 10 degrees of static advance, if the maximum advance required is 38 degrees, then it's range is 28 degrees (38-10), this means that if the static setting is increased to 18 degrees, then the total advance will be 46 degrees (18+28), way too much. It is unlikely that the standard distributor will give the correct amount of advance, it will usually give too much. This is why we must restrict the total centrifugal advance that the distributor is capable of supplying to our new figure, in this case 20 degrees, then with the static setting of 18 degrees, the maximum advance will be 38 degrees (18+20), the correct figure.
If the advance supplied is MORE than required, and this is highly likely, it means as expected that the distributor is supplying too much mechanical advance, and that the stops in the distributor must be bent to restrict the travel of the mechanism. If the advance supplied is LESS than required which is unusual, then the distributor is supplying too little mechanical advance and the stops must be filed to allow more travel of the advance mechanism.



OK..So that is all great...But how do we make the adjustments they describe?????

Lets have a look at it.....

The Ford Duraspark distributor is an inexpensive and reliable alternative to the expensive aftermarket distributors. You can find them for about $52.00 US retail and the recurve spring kits are cheap at about $7.00 US. This recurving operation usually takes me about an hour or less. Just use care with the little parts as they like to migrate out of sight or flip half way across the room.


Step 1
Remove the two vacuum advance dashpot to main body screws and then the vacuum advance arm to breaker plate pin C clip


Pull the advance dashpot away from the main body and pull the arm off of the breaker plate pin.



Step 2
Mark reluctor position as there are two slots.
Remove reluctor from upper advance shaft. Use two pry bars or large screwdrivers and Do Not pry on the teeth but rather the beefy portion of the reluctor near the center shaft.





Step 3
Remove the two breaker plate to main body retaining screws. Rotate the pick up to gain access. lift off breaker plate.



Step 4
Decide which slot you want to use and if necessary remove the upper advance shaft retaining clip and springs so you can lift and rotate the shaft in to the proper position. If ordering a distributor from a parts store ask for one for a 1975 elite with a 460 4v engine. You will be more likely to get the lower number advance slot like a 10L or 13L rather than the 18 or 21L's. If you have a distributor with the larger numbered slots limit rotation via welding the slot smaller or placing a bushing around the pin to limit total shaft rotation.



To figure approximate slot width for a given advance figure Multiply the number of desired centrifugal degrees by .013" then add .150" to account for the width of the stop pin.

8L slot = 16 degrees centrifugal advance = .358”
9L slot = 18 degrees centrifugal advance = .384”
10L slot = 20 degrees centrifugal advance = .410”
11L slot = 22 degrees centrifugal advance = .436”
12L slot = 24 degrees centrifugal advance = .462”
13L slot = 26 degrees centrifugal advance = .488”
14L slot = 28 degrees centrifugal advance = .514”

Step 6
install the springs from the recurve kit and bend the tabs to keep tension on both springs so the advance shaft returns to its idle position



Step 7
Reassemble in the reverse order. Breaker plate, Reluctor, Vac advance dashpot and return the little clips to their proper place.



Place the reluctor back on the shaft in its original position and align the roll pin slot in the reluctor with the roll pin slot on the upper distributor shaft. Carefully place the roll pin into place after fully seating reluctor on shaft and gently push it back into place with a hammer and a small punch. The 1/2 slots in both the reluctor and in the shaft together make a round hole for the roll pin.

So what about Vacuum Advance?



Vacuum Advance
Under conditions of light or closed throttle, the volumetric efficiency of an engine is quite poor, and cylinder filling is affected to the extent that the effective compression ratio is much lower than the static or calculated compression ratio. In these circumstances the mixture will burn much more slowly than with a fully filled cylinder and the flame front will reach the piston quite late. This can dramatically cut the overall efficiency of the engine and its economy. Under these conditions the engine will tolerate and indeed benefit from advancing the timing by up to 15 degrees over its normal setting.
The device that usually performs this trick is called the vacuum advance device. The way this works is to exploit the partial vacuum that is present in the inlet manifold when the throttle is closed or partly closed. A tube is connected from the manifold to a sealed diaphragm in the distributor, which in turn is connected to the distributors base plate. The suction deflects the diaphragm which turns the base plate against the direction of rotation of the distributor thereby advancing the timing, this gives much better throttle response on part throttle, and far better economy.
Many people who tune engines disconnect the vacuum advance mechanism, and indeed on some distributors it is very hit and miss in operation and can cause anomalies in the timing. All in all however for a road engine, the vacuum advance retard should be retained if it is possible to do so (not always easy with sidedraught carbs). This will have a dramatic affect on economy and driveability especially on small throttle openings and when 'off-cam'.

Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpms, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

In short, vacuum advance was developed to optimize fuel economy and reduce emissions. It is not a bad thing to have on a car which sees a lot of street driving, and in such conditions the engine will perform better with it properly adjusted. However many factory and aftermarket performance distributors do not even come with a vacuum advance. The reason is simply because race cars do not spend much time at part throttle.

Setting Vacuum Advance:



Vacuum advance canisters are usually adjustable with a 3/32-in. allen wrench, as shown here. The small screw inside the housing adjusts the tension on the diaphragm spring. If you detect knocking and loss of power, back the screw out (counterclockwise) to decrease advance. If the engine pops and surges, tighten up the screw to increase advance.

Note:When checking initial and total advance, always disconnect the vacuum advance hose. Otherwise you will get very high timing readings.

Tuning Vacuum Advance
The last step, after the total advance curve is set, is to dial in the vacuum advance if you have one. There should be a vacuum line connected from the carb, or the manifold, to the vacuum canister. There are two types of vacuum sources that you should be aware of. One type is known as "full" vacuum or "manifold" vacuum. This is a direct connection to the manifold, and if the hose is connected to this port you will get vacuum in the line at idle. The other port is a "timed" port, which only yields a vacuum above a certain rpm. At idle the line will have no vacuum. Most carburetors have both ports. On Holley's the timed is above the throttle blades, and the "full" is below, near the base. On Carter/Edelbrock carbs, the timed port is on the passenger side and the full is on the driver's side. The easiest way to confirm what port you have is to hook up a vacuum a gauge and check for vacuum at idle. The preferred vacuum source is the timed source. This way there is no effect on the initial timing setting.


Remember vacuum advance is load dependent, so you cannot check it with a timing light with the car in neutral. The best way to set vacuum advance is by feel, under real driving conditions. Connect the vacuum line and drive the car up a steep, long grade, with the car in high gear and at a low speed, 30-40mph. Occasionally push the accelerator to the floor, and listen and feel for knocking and/or loss of power. If you detect this, immediately back of. This means the canister is advancing too much and you should adjust the canister so the diaphragm is 'tighter', by turning the screw counterclockwise.

You can also adjust the vacuum canister clockwise until it does start to ping and then back it off 2 turns. This should set your Vacuum Advance perfectly.

Well I hope this helped get all the info you need in one place!! I know I had a hell of a time to get everything together to accurately set my Ignition Timing.

If you see anything that needs to be clarified or is misinformed...Please send me a PM and I will correct it.

Cheers!!!